The present invention pertains to the field of cryogenic air separation, and in particular to a process for the delivery of oxygen at a variable flow rate from a distillation column system.
The ability to supply oxygen to a customer at widely varying rates has always been particularly important in some industry sectors such as steel production and integrated gasification combined cycles (IGCC) for electricity generation. The importance of this ability has grown recently for other sectors due to the trend in industrial gas producers taking advantage of time-of-day and other types of contracts to reduce their operating costs. In such situations, the response time of a cryogenic air separation unit can be much slower than that necessary to meet variable demand rates. This is particularly true when oxygen is produced from a double column distillation configuration. It is thus advantageous to isolate the distillation columns from disturbances by withdrawing oxygen at a constant rate which corresponds to the time-average production. In such an event, any excess oxygen product must be stored temporarily during periods when the customer demand is reduced relative to the time-average production and oxygen product must be withdrawn from storage when the customer demand exceeds the time-average production.
The prior art has suggested storing oxygen as a compressed gas in high pressure storage bottles. This technique is useful when the variations in customer demands are of high frequency and/or of short duration. However, due to the high pressures and volumes necessary to store product in the gas phase, it generally is much more economical to store product in the liquid phase.
Storing product in the liquid phase, however, also has at least one disadvantage. Since the product is required in the vapor phase by the customer, the liquid must be vaporized in accordance with variable demand rates. Since oxygen often is vaporized by heat exchange with an incoming warm stream, such as air, the variable rate of oxygen vaporization produces a variable rate of liquid feed to the distillation columns. Such variations constitute disturbances which can affect oxygen product purity.
According to the prior art, by providing storage for the incoming liquefied feed and storage for the outgoing liquid oxygen product, the flow rates of the liquefied feed and the products of the columns can be held essentially constant by allowing the inventories in the feed and the product storage tanks to vary. U.S. Pat. No. 5,082,482 (Darredeau) teaches transferring all of the liquefied air to a storage vessel, withdrawing the liquid air at a constant rate from the storage vessel, and transferring the liquid air to the distillation system. The liquid air storage operates at a pressure slightly greater than the pressure of the distillation system.
U.S. Pat. No. 5,265,429 (Dray) teaches a variation on Darredeau whereby only a portion of the liquid air is directed to storage during periods of high oxygen production, and liquid air is transferred from storage to the main liquid air circuit during periods of low oxygen production. In either event, the storage vessel must operate at a pressure greater than that of the distillation system. U.S. Pat. No. 5,526,647 (Grenier) teaches the use of a storage vessel for liquid air that is maintained at pressures substantially greater than the pressure of the distillation system.
All of the prior art patents teach methods wherein both the inventories of the incoming liquefied air and the outgoing liquid oxygen are varied so as to allow the feed flow rate to, and the product flow rate from, the distillation columns to remain essentially constant. These patents also teach that the liquid air fed to either the higher pressure column, lower pressure column, or both columns is extracted from the liquid air storage vessel.
The disadvantages of storing the liquid air at pressures greater than that of the distillation system depend on the degree to which the pressure is greater. The pressure of the main liquid air stream often is 200 psia to 1200 psia. If the liquid air storage pressure is maintained at that of the incoming liquid air, the storage vessel must be capable of withstanding high pressure and consequently is expensive to construct. If the liquid air storage pressure is less than that of the main air, then the fluid entering the storage vessel may produce vapor upon pressure reduction. This flash vapor must be routed to the distillation system at a variable rate, since the liquid air flow sent to the storage vessel is variable. Since the variation in vapor flow resulting from the liquid air pressure reduction is small compared to the vapor flows in the distillation system, the resulting impact on product purity can be minimized through appropriate control strategy. However, the variation in vapor flow at the liquid air storage vessel itself can be large in relative terms. This makes it difficult to control storage pressure which in turn impacts the pressure or flow of liquid air into storage. Thus, storing liquid air at a pressure intermediate of the main liquid air and the distillation system does not completely eliminate disturbances.
U.S. Pat. No. 5,084,081 (Rohde) teaches a method of withdrawing and storing a nitrogen-rich liquid and oxygen-enriched bottoms from the higher pressure column at a variable rate and introducing streams of the nitrogen-rich liquid and the oxygen-enriched bottoms at a constant rate to the lower pressure column. This maintains constant rates in the lower pressure column but allows flow variations in the higher pressure column. The system taught by this patent requires three storage vessels--one for liquid nitrogen, one for liquid oxygen, and one for liquid oxygen-enriched bottoms.
It is desired to have a more operable process for the delivery of oxygen at variable flow rates.
It also is desired to have a process for the delivery of oxygen at a variable flow rate which overcomes the difficulties and disadvantages of the prior art to provide better and more advantageous results.